In recent years, double layer optical recording medium having two layers of recording film have been proposed. A double layer optical recording medium is provided with an extremely thin semi-reflective film having a thickness of about 5 to 15 nm on the side of incident ray. This semi-reflective film not only functions as a reflective film for the recording layer on the side of incident ray, but also exhibits the function of transmitting laser beam to record or replay on the second recording layer. As the semi-reflective film for a double layer optical recording medium, a pure Ag film or an Ag alloy film has been used. A pure Ag film or an Ag alloy film is widely used because it not only allows heat to rapidly escape from the recording film, but also exhibits low absorptance to laser beams in a wide wavelength range of 400 to 800 nm.
As an example of a reflective film for an optical recording medium, a reflective film for an optical recording medium is known which has a composition consisting of 0.001 to 0.1% by mass of Ca and a remainder containing Ag, and this reflective film is formed by sputtering an Ag alloy target having a composition consisting of 0.001 to 0.1% by mass of Ca and a remainder containing Ag. This reflective film having a composition consisting of 0.001 to 0.1% by mass of Ca and a remainder containing Ag exhibits substantially the same reflectivity as a pure Ag reflective film, and is superior to a pure Ag film in that recrystallization is suppressed, and the reflectivity thereof does not decrease with lapse of time (see Patent Document 1).
Further, as another example of a reflective film for an optical recording medium, Patent Document 2 discloses a semi-reflective film which has a composition consisting of 0.1 atom % or more of at least one member selected from the group consisting of Nd and Y, 0.2 to 5.0 atom % of at least one member selected from the group consisting of Au, Cu, Pd, Mg, Ti and Ta, and a remainder containing Ag, and this semi-reflective film is formed by sputtering an Ag alloy target having a composition consisting of 0.1 atom % or more of at least one member selected from the group consisting of Nd and Y, 0.2 to 5.0 atom % of at least one member selected from the group consisting of Au, Cu, Pd, Mg, Ti and Ta, and a remainder containing Ag.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2003-6926
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2002-15464